Generally, a washing machine serves to forcibly move laundry inside a drum which rotates by receiving a driving force of a driving motor, using a mechanical force such as a frictional force between the drum and the laundry, in a state where a detergent, washing water and the laundry have been introduced into the drum. As the laundry receives a physical force such as friction or impact, a washing operation is performed. The washing operation may be performed by a chemical reaction through a contact between the laundry and the detergent. The chemical reaction of the detergent may be accelerated as the laundry moves in the drum.
In a drum type washing machine much used recently, a rotation shaft of a drum is formed in a horizontal direction. The rotation shaft of the drum may be inclined from the horizontal direction. In such drum type washing machine where the rotation shaft is formed in a horizontal direction, laundry moves in a circumferential direction along an inner circumferential surface of the drum.
Laundry moves along the inner circumferential surface of the drum by a centrifugal force due to rotation of the drum, and by friction with the inner circumferential surface of the drum. A lifter, configured to help such movement of the laundry, may be provided on the inner circumferential surface of the drum. According to a rotation speed of the drum, the laundry may perform a circular motion along the inner circumferential surface of the drum, or may perform a falling motion from an upper side of the drum by gravity. Such falling motion greatly influences on a washing effect.
Movement of the laundry in the drum greatly influences on a washing effect. More specifically, various movements of the laundry result in a uniform washing operation by changing contact surfaces between the laundry and the inner circumferential surface of the drum. Further, a washing effect can be enhanced by increasing a physical force applied to the laundry.
In the conventional drum type washing machine, a single drum is rotated to move laundry. The laundry can move only in a circumferential direction of the drum along the inner circumferential surface of the drum on the initial position. That is, the laundry cannot perform a complicated movement such as a movement in a shaft direction or a rotation movement. The laundry can perform only a 2D movement. The reason is because an additional external force to generate a complicated movement of the laundry is not generated. Under such configuration, the movement of the laundry is limited, and thus the washing effect is limited. Further, a washing time (duration) and power consumption are increased.